1. Field of the Invention
The present invention relates to thin GaN film-joined substrates formed of a substrate different in type, or chemical composition, from GaN, and a thin film of GaN joined on the substrate different in type, and methods of producing the same. Furthermore the present invention relates to GaN-based semiconductor devices including at least one GaN-based semiconductor layer deposited on the thin film of GaN and methods of fabricating the same.
2. Description of the Background Art
A GaN substrate is suitably used as well as GaAs, InP and other similar substrates for semiconductor devices. The GaN substrate, however, requires an extremely higher fabrication cost than GaAs and InP substrates. Accordingly semiconductor devices utilizing a GaN substrate require an extremely high fabrication cost. This derives from a difference between the method of producing a GaN substrate and those of producing GaAs and InP substrates.
More specifically, for GaAs and InP substrates, Bridgman crystal growth, Czochralski crystal growth or similar liquid phase deposition is employed to grow a crystal. As such, the crystal can be grown fast. For example, at least 200 mm thick, large GaAs and InP bulk crystalline bodies can be readily grown in a period of approximately 100 hours, and such thick, large bulk crystalline bodies allow approximately 200 μm to 400 μm thick, free standing GaAs and InP substrates to be cut out therefrom in large amounts, respectively, (e.g., at least 100 substrates for each type of substrate).
For a GaN substrate, in contrast, hydride vapor phase epitaxy (HVPE), metalorganic chemical vapour deposition (MOCVD) or other similar vapor phase deposition is employed to grow a crystal. As such, the crystal is grown slowly. For example, a crystal growth period of approximately 100 hours can only provide an approximately 10 mm thick GaN bulk crystalline body, and such bulk crystalline body only allows a small amount of (e.g., approximately 10) approximately 200 μm to 400 μm thick, free standing GaN substrates to be cut out therefrom.
If the GaN film to be cut out from the GaN bulk crystalline body is reduced in thickness to provide an increased number of GaN substrates, however, the film is reduced in strength and cannot be a free standing substrate. Thus there is a need for a method reinforcing the thin film of GaN cut out from the GaN bulk crystalline body.
One such method reinforcing the thin film of GaN is to produce a thin GaN film-joined substrate formed of a substrate different in type, or chemical composition, from GaN, and a thin film of GaN joined on the substrate different in type (hereinafter also referred to as a “film-joined substrate”). Such film-joined substrates are produced in methods disclosed for example in Japanese Patent National Publication No. 2004-512688 and Japanese Patent Laying-open No. 2005-252244. If film-joined substrates produced in the methods described in the publications are used to fabricate semiconductor devices by MOCVD, MBE or similar vapor deposition, however, the thin film of GaN deposited on the substrate different in type disadvantageously peels off the substrate in the step of depositing a semiconductor layer on the thin film of GaN.